I imagine the kind snd and brand of b12 matters? Which do you use?
When I first took B-12, it many years ago and was based on reading a professional level medical text article about the signs of a B-12 deficiency. I had developed fatigue, mild verbal aphasia, and short term memory problems that my doctor blithely dismissed as due to stress and over-work.
My reaction to several tablets of B-12 within a few hours on a Friday evening was such that I barely slept for the next three days due to a sudden burst of energy and mental activity.
The cause of the B-12 deficiency was unrecognized celiac disease, with a tooth extraction that required nitrous oxide gas probably aggravating that preexisting condition. A few months after taking the B-12, I stopped eating gluten and my health improved dramatically.